The present invention relates to microelectromechanical Systems (MEMS). More particularly, the present invention pertains to frequency selective MEMS devices.
Currently, there is an interest in increasing the degree of integration of electronics. Integration has proceeded steadily over the last few decades and achieved remarkable reduction in the physical size occupied by electronic circuits. Semiconductor lithography has enabled circuits with millions of transistors to be constructed on a single silicon die. Nonetheless, certain components are difficult to integrate.
One important component that is used to generate stable frequencies in a variety of electronic apparatus including sequential logic (e.g., microprocessors) and wireless communication transceivers is the quartz crystal resonator. The quartz crystal resonator in its usual form is a bulky discrete component.
Microelectromechanical System (MEMS) based resonators have been proposed as alternatives to quartz resonators for use as frequency selective components at RF frequencies. One type of MEMS resonator that has been proposed comprises a suspended beam of semiconductor material that is shaped and sized to resonate at a selected frequency chosen in view of a desired electrical frequency response. The MEMS resonator serves as a frequency selective component in a circuit. According to one design the MEMS resonator is driven by a drive electrode that extends below the suspended beam. Electric force interaction between the suspended beam and the drive electrode induces the suspended beam to vibrate. Similar structures may be used as frequency selective filters. In such a use an input signal applied to a first terminal is used to drive the mechanical structure of the resonator into resonance, and an output signal is coupled out of a second terminal. In as much as only that part of the signal is near the resonant frequency of the resonator, only a narrow frequency band of the applied signal is coupled to the second terminal.
During the past decade there has been an increased interest in the semiconductor industry in the use of Silicon-On-Insulator (SOI) wafers. SOI wafers include a silicon substrate, a silicon di-oxide layer on the silicon substrate, and a single crystal silicon layer on the silicon di-oxide layer. SOI wafers afford a number of advantages in terms of the electrical properties of circuits built using them, including reduced voltage requirements, and power consumption for a given clock speed.
In a previously filed patent application entitled xe2x80x9cMEMS RESONATORS AND METHODS FOR MANUFACTURING MEMS RESONATORSxe2x80x9d Ser. No. 09/828,431 (Application pursuant to Motorola disclosure numbers: CM03351J, CM03352J, CM03524J) filed on Apr. 9, 2001 and assigned to the assignee of the present invention, a type of MEMS resonators that is fabricated on SOI wafers is disclosed.
In the disclosed SOI MEMS resonators, a flexural mode resonant beam and a number of support beams that attach to the flexural mode resonant beam at node points are etched from the top single crystal silicon layer of the SOI wafer. A portion of the silicon di-oxide layer in an area underneath the flexural mode resonant beam, and the support beams is removed by an isotropic etch to allow for free movement of the flexural mode resonant beam and the support beams. For the disclosed types of MEMS resonators, at least some of the fabrication steps required to fabricate the resonator, may be accomplished by processing operations (e.g., resist exposure, doping, etching) that are also conducted for the purpose of fabricating electrical circuits on the die on which the MEMS resonators are fabricated. Thus, the disclosed MEMS resonators may be integrated with electronic circuits very efficiently.
The frequency of resonators used in electrical circuits such as oscillators is often specified at a precision of tens of parts per million. For filtering applications even higher degrees of accuracy are desired. On the other hand the dimensional tolerances that are achieved semiconductor lithography and etch processes are often on the order of plus or minus 5% percent. Dimensional variations of resonators fabricated using semiconductor lithography may, consequently, suffer wide variations in resonant frequency.